A new study found that more than a third of those who recovered from an infection had symptoms recur days or weeks later.

When the antiviral treatment Paxlovid came into wider use for Covid-19 infections earlier this year, doctors who prescribed it and patients who took it noticed that symptoms sometimes flared up again a few days after having gone away. Some people even tested negative before they experienced the rebound. But this puzzling phenomenon can occur whether you take Paxlovid or not, according to a new study. Researchers found that when patients received a placebo instead of treatment, a portion of them still experienced a rebound of their symptoms after they had initially improved.

To understand the natural variability in coronavirus symptoms, Dr. Smith and his team tracked 158 clinical trial participants who had tested positive for Covid from August to November 2020. Each person kept a daily diary and marked 13 different Covid symptoms as being absent, mild, moderate or severe. Among the 108 people whose symptoms had improved without antiviral treatment, and had completely disappeared for at least two consecutive days, 48 people (44 percent of those who recovered) noted that symptoms flared up again at various times during four weeks of follow-up.